1. Field of the Invention
This invention relates to a heat insulation cylinder for use in a silicon single crystal pull-up apparatus, a silicon single crystal pull-up apparatus using such a heat insulation cylinder, heat insulation material used for the above-mentioned heat insulation cylinder, as well as a process for producing such material.
2. Description of The Related Art
Silicon single crystals used as substrates for semiconducter devices have been mainly prepared by Czochralski (CZ) method. In the CZ method, an ingot of single crystal is prepared, in principle, by rotatably supporting a crucible in a chamber, charging a silicon material in the crucible, melting the silicon material in the crucible by a carbon heater disposed at the outer circumference of the crucible, dipping a seed crystal rotatably suspended from above the molten silicon liquid to the surface of the molten silicon liquid and then pulling up the same.
Conventional heat insulation cylinders prepared by winding felt-like woven cloth made of carbon fibers in a plurality of layers to the outer circumference of a carbonaceous cylindrical plate is disposed around the outer circumference of the carbon heater. The heat insulation material used for the heat insulation cylinder has a porosity of about 85%, an intrinsic specific gravity of about 1.70 and a bulk specific gravity of about 0.4.
However, in the case of using the heat insulation cylinder molded by cylindrically winding the woven cloth of carbon fivers, since the effect of preserving the heat generated from the heater is low, the heater output has to be increased. Further, carbon fibers are oxidized and degraded thereby liable to be detached during long time use thus bringing about contamination by the intrusion of carbon powder into the crucible, causing shrinkage of the woven cloth and reducing the effect of insulating heat transmitted to the chamber. Accordingly, a great amount of flow rate is required as the cooling water for the chamber. This leads to a particularly serious problem under the present situation where the diameter of the crucible increases along with the increase in the diameter of the silicon single crystal ingot and the output of the heater has to increase in order to melt the silicon material.
Further, no sufficient heat insulation effect can be obtained with the heat insulation material having the porosity, the intrinsic specific gravity and the bulk specific gravity as described above.
By the way, a crucible made of quartz glass is practically used as the crucible. The quartz crucible reacts with the molten silicon liquid to generate SiO gas as shown below: EQU SiO.sub.2 +Si.fwdarw.2SiO
The SiO gas reacts with carbon heated to the highest temperature to generate CO gas as shown below: EQU SiO+2C.fwdarw.SiC+CO
As a result, the working life of the carbon heater was shortened. Further, since CO gas is intaken into the molten silicon liquid, the carbon concentration in the ingot of the silicon single crystal increases and the crystal defects due to the increasing of carbon concentration increase.